Apply Petrol/Volt/Kupfer design system and project sidebar

Bring the Econsult-family design tokens (see design-system-econsult.md)
into leistungsbilanz-ts and add a persistent sidebar for the projects
area, mirroring elt-planung-suite / Einsatz-Orga. The sidebar lists all
projects and, inside a project, jumps to its sections; the tree editor
stays sidebar-free since it needs the full width. The projects overview
page is decluttered to a single stacked column (create/import/global
devices) now that the project list lives in the sidebar.

src/frontend/components/AppShell.tsx

@@ -0,0 +1,22 @@
+"use client";
+
+import { usePathname } from "next/navigation";
+import { Sidebar } from "./Sidebar";
+
+const PROJECT_AREA_PATTERN = /^\/projects(\/[^/]+)?$/;
+
+export function AppShell({ children }: { children: React.ReactNode }) {
+  const pathname = usePathname();
+  const showSidebar = PROJECT_AREA_PATTERN.test(pathname ?? "");
+
+  if (!showSidebar) {
+    return <>{children}</>;
+  }
+
+  return (
+    <div className="app-shell">
+      <Sidebar />
+      <div className="app-shell-content">{children}</div>
+    </div>
+  );
+}